Most Revd Justin Welby writes of the "troubled, uncertain world" we live in and the need to stand alongside fellow believers as they endure suffering.
Speaking of the treatment by extremist groups, he said: "In some places, this is motivated by a desire to eradicate the indigenous Christian presence completely. These are acts not only of terror but of genocide; criminal acts for which the international community must bring those guilty to account.
"Yet although so vulnerable and often forgotten and marginalised, our brothers and sisters are being courageous in the Lord. Indeed, 'God chose what is weak in the world to shame the strong' (1 Corinthians 1.27).
"In other places conflict and corruption have become so normal that the world forgets the suffering of the poor.
"I ask your prayers for those of us who live in safety that we may not be bystanders afar off, beating our breasts as we retire to the security of our homes, but that we may draw nearer to the cross of Jesus, stand there alongside our suffering brothers and sisters and be ready to take our part in practical action for change.
"I pray that Christ will strengthen all his people in our inner being with power through the Holy Spirit to be faithful, to have courage and to live in hope."
There have been many calls for the suffering experienced by Christians at the hands of Islamic State to be labelled genocide.
Earlier this year the UK parliament voted to use the word and called on the government to refer the issue to the UN Security Council.
Once referred, member states then have a legal duty to take action to stop the genocide.
The UK government is yet to refer the issue.
Speaking during the vote, Foreign Office Minister Tobias Ellwood said it was up to the courts rather than governments to bring about justice.